Embodiments of the invention relate generally to radiographic detectors for diagnostic imaging and, more particularly, to a Computed Tomography (CT) detector module having a multi-faceted construction that provides for increased slice acquisition with minimal image data degradation.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector and rejecting scatter from the patient, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
In the last decade, the development of volumetric or cone-beam CT (VCT) technology has led to a rapid increase in the number of slices (Z-axis) used in CT detectors. Indeed, the detectors used in VCT are enabling more and more coverage in patient scanning by increasing the patient area exposed. In order to accommodate such coverage, the width of CT detectors has been increased in the Z-axis (i.e., direction of patient length). The x-ray detectors of current state of the art CT systems are composed of a 2D array of scintillating pixels, coupled to a 2D array of silicon photodiodes, with the typical array being sized so as to be capable of providing for acquisition of 64 slices (i.e., array size of 40 mm at ISO in case of GE scanner).
Recently, however, the need for cardiac imaging has become more and more of interest and imaging of the heart within one rotation has become a requirement. In order to image the heart in one rotation, the detector array size needs to be ˜160 mm at ISO to cover the full organ in half scan, which is equivalent to a detector of 256 slices in our case. However, increasing the coverage of the detector in the Z-axis beyond 64 slices up to 256 slices might lead to a degradation in performance of the detector. That is, the performance of the detector pixels, especially those pixels at a greater distance from a centerline of the detector along the Z-axis, will be degraded because of the angle at which such pixels receive x-rays from the cone beam. At a certain position along the Z-axis, the primary beam of x-rays will cross two contiguous pixels in the Z-direction, thereby inducing a significant crosstalk from slice to slice, spectral non-linearity because of the beam hardening with the pixels, slice profile degradation, and Modulation transfer function (MTF) deterioration, collectively known as “parallax.” This parallax caused by the increased number of slices can lead to artifacts being present in the reconstructed CT image, thereby presenting a significant drawback to image quality provided by a 256 slice detector and beyond.
Therefore, it would be desirable to design a CT detector that provides for VCT cardiac imaging by accommodating data acquisition of up to 256 slices. It would also be desirable for such a CT detector to minimize the parallax effect in such a detector, so as to provide for high quality image reconstruction of the cardiac region of a patient.